<div id="imprime">
   <div id="fondo">
      <div class="letra">
         Hola aca mi prueba
      </div>
   </div>        
</div>
<?php echo $this->Form->button('Imprimir', array('title'=>'Imprimir lista', 'id'=>'btnImprimir')); ?>
 <script type="text/javascript">
   jQuery(document).ready(function() {

         jQuery("#btnImprimir").click(function() {
            //printElem({ leaveOpen: true, printMode: 'popup' });
            printElem({ overrideElementCSS: ['http://localhost/inventario/app/webroot/css/prueba.css'] });
         });


     });
 function printElem(options){
     jQuery('#imprime').printElement(options);
 }
</script>